Deterministic and ensemble forecasts of the Kuroshio south of Japan

Kuroshio flows eastward along the southern coast of Japan and has a variety of flow paths such as straight and large meander paths south of Japan. The Kuroshio path variations cause substantial damage to fisheries, marine transport, and marine environment (e.g., Nakata et al. 2000; Barreto et al. 2021). Consequently, Japanese research institutions have conducted Kuroshio path predictions using regional ocean data assimilation systems with the Kalman filter (Hirose et al. 2013) and the three- and four-dimensional variational methods (Miyazawa et al. 2017; Kuroda et al. 2017; Hirose et al. 2019). However, these systems are not designed for ensemble forecasts, and the predictions have been limited to deterministic ones so far.

We have developed a new local ensemble transform Kalman filter (LETKF)-based regional ocean data assimilation system (Ohishi et al. 2022a, b) and released ensemble ocean analysis datasets called the LETKF-based Ocean Research Analysis (LORA) for the western North Pacific and Maritime Continent regions (Ohishi et al. 2023, 2024a, b). The LORA datasets are shown to have sufficient accuracy for geoscience research, especially in mid-latitude regions (Ohishi et al. 2023), and we can perform both deterministic and ensemble forecasts initialized by the LORA. Therefore, this study aims to compare the predictability of the Kuroshio path south of Japan between deterministic and ensemble forecasts.

We performed 6-month deterministic and ensemble forecasts initialized on the first day of every month from January 2016 to December 2018 (36 cases in total) using the initial conditions of the analysis ensemble mean and 128 analysis ensembles from the LORA dataset, respectively. The results show that the predictability limits of the Kuroshio path are 74 and 108 days in the deterministic and ensemble forecasts, respectively, indicating a significantly longer predictability limit of the ensemble forecasts than the deterministic forecasts.